Grakn needs to be able to connect to all nodes in a Cassandra cluster. Could you check that you have connectivity from the Grakn server to all Cassandra nodes?
Please also make sure you are running Cassandra 2.1.x (2.1.17 preferable) which has the thrift interface enabled by default.

If that doesn’t resolve the issue, can you give more detail on how your Grakn and Cassandra cluster are set up? If possible, please provide us with the configuration files for both.